Serena Williams has followed Victoria Azarenka in withdrawing from the Dubai Duty Free Tennis Championships.

A foot injury forced Azarenka out of the tournament on Tuesday and Williams has also been forced out on account of injury.

The world No. 1, who returned to the top of the rankings this week, had been nursing a lower back problem for a few weeks and it was troubling her on Wednesday morning.
And under advice from medical staff, the American elected to withdraw prior to her clash with Marion Bartoli.

Bartoli will now meet Caroline Wozniacki in the next round.

Wozniacki has shown signs of returning to form of late and the Danish former world No. 1 was mighty impressive in destroying Jie Zheng 6-0 6-1.

Serving has been a bit of an issue for Wozniacki, but she converted 68% of first serves and did not send down a single double fault in the 54 minutes on court. She was also ruthless when given the chance, as she converted all five break points.

Defending champion Agnieszka Radwanska was made to fight to claim a 7-5 6-3 win over Yulia Putintseva.

"It is my first match here so I am happy to win the match in two sets," Radwanska said. "She played well and was moving very well, mixing it up, and it was tough."

Up next for Radwanska is Petra Kvitova who has a narrow 7-5 7-6(1) winner over Ana Ivanovic.

Nadia Petrova took only 75 minutes to see off Carla Suarez Navarro 6-1 6-0, while the seventh seed Sam Stosur was a 6-4 6-0 winner over Hsieh Su-wei.

Angelique Kerber's form has been patch of late and the German No. 4 seed was completely outplayed by Roberta Vinci who eased to a 7-5 6-1 win.